Method for producing a dielectric interlayer and storage capacitor with such a dielectric interlayer

ABSTRACT

A dielectric interlayer, especially for a storage capacitor, is formed from a layer sequence subjected to a temperature process, wherein the layer sequence has at least a first metal oxide layer and a second metal oxide layer formed by completely oxidizing a metal nitride layer to higher valency.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.11/438,700, filed May 23, 2006, now U.S. Pat. No. 7,416,952, which isherein incorporated by reference in its entirety.

TECHNICAL FIELD OF THE INVENTION

The invention relates to a method for producing a dielectric interlayer,a method for producing a storage capacitor and a storage capacitor withsuch a dielectric interlayer, especially for use in a semiconductorstorage cell, which is part of a semiconductor memory. The semiconductormemory is preferably a dynamic random access write-read memory (DRAM).

BACKGROUND OF THE INVENTION

Semiconductor memories, primarily DRAMs, are usually realized as astorage cell array on a semiconductor wafer. The storage cells comprisesa storage capacitor and a selection transistor. In a read or writeoperation, respectively, the storage capacitor is charged or discharged,respectively, with an electrical charge corresponding to one data unit(bit) through the selection transistor. For this, the selectiontransistor is addressed through a bit- or wordline, respectively, by theaid of a peripheral logic having switching transistors.

A substantial center in the technological development of thesemiconductor memories is the storage capacitor. In order to provide forsufficient storage capacity with small cross-sectional area, the storagecapacitors are therefore realized in three-dimensional manner. Asessential embodiments of three-dimensional storage capacitors, trenchcapacitors and stacked capacitors have gained acceptance. In trenchcapacitors, a trench is etched into the semiconductor substrate, whichis filled with a dielectric interlayer and a first storage electrodelayer, wherein a doped region of the semiconductor substrate around thetrench serves as a second storage electrode layer. The selectiontransistor of the storage cell is usually formed on the semiconductorsurface besides the trench capacitor as a planar field effecttransistor, wherein the one transistor electrode is connected to the oneelectrode layer of the trench capacitor.

In contrast, stacked capacitors are formed on the surface of thesemiconductor substrate, wherein a first storage electrode layer isrealized in form of a crown, which is separated from a second storageelectrode layer via a dielectric interlayer. The selection transistor ofthe storage cell is disposed in form of a planar field effect transistorbelow the stacked capacitor, wherein the one transistor electrode isconnected to the crown-shaped storage electrode layer of the stackedcapacitor.

Due to the continuously increasing downsizing of the semiconductorstorage cells, also in three-dimensional storage capacitors, it issearched for additional possibilities to lower the area requirement andto increase the capacitor capacity at the same time.

Conventionally, in storage capacitors, material combinations of silicaand/or silicon nitride are employed as the dielectric interlayer.However, for sub 100 nm structures, it is contemplated to replace theconventionally employed silica and/or silicon nitride layers bymaterials exhibiting a higher dielectric constant and thus allowincrease of the storage capacity per unit area. As such so-called high kdielectrics, especially binary oxides such as alumina, tantalum oxide,hafnium oxide, zirconium oxide, oxides of the lanthanum group, aluminacompounds and further single and mixed oxides are contemplated.

However, many of the contemplated high k dielectrics can only verydifficultly be integrated into the standard process for producingstorage capacitors within the bounds of the silicon planar technique,and especially only difficultly be formed as extremely thin layers.Further, the dielectric strength of many contemplated high k dielectricsis insufficient for employment in DRAM storage capacitors, especiallyconcerning the long term stability. Furthermore, is has turned out thatin many of the considered high k dielectrics, increased leakage currentsoccur with respect to the conventional material combinations of silicaand/or silicon nitride, which entail shortened retaining time of thecharge in the storage capacitor.

In employment of high k dielectrics within the bounds of the siliconplanar technique, further, it has turned out that such layers result inhigh tensile stresses on the semiconductor surface, which in turnentails bending the semiconductor wafer. In employment ofthree-dimensional storage capacitors within the bounds of the DRAMproduction, due to the increased surface, therein, bends of several 100μm can occur, whereby further processing the semiconductor substrate fordevice formation within the bounds of the silicon planar technique,wherein layers have to be applied consecutively precise in position,becomes nearly impossible. There is also the danger that thesemiconductor wafer breaks due to the high stress.

These disadvantages especially apply to the employment of alumina(Al₂O₃) as a high k dielectric in storage capacitors, the preferredcandidate for replacement of the conventional material combinations ofsilica and/or silicon nitride. Alumina distinguishes itself by beingable to be relatively simply integrated into the standard process forproducing storage capacitors within the bounds of the silicon planartechnique. In producing storage capacitors with alumina as thedielectric interlayer within the bounds of the silicon planar technique,on a first capacitor electrode, which usually is a highly doped siliconlayer, the alumina is deposited, which is subsequently thermallyconcentrated with a high-temperature process for improving thedielectric strength, for reducing the leakage current and for increasingthe dielectric constant. Then, a second capacitor electrode layer,preferably a metal layer, is applied onto the alumina layer. In order toachieve sufficient dielectric strength with not too high leakage currentat the same time, the concentrated alumina layer has to have a thicknessof at least 5 nm, which results in high tensile stress on the substratesurface, which causes overlay problems in the subsequent processing.

With the employment of alumina, the dielectric constant of which is at10, an increase of the storage capacity per unit area can be achievedover the conventionally employed dielectric interlayers of a materialcombination of silica and/or silicon nitride. However, compared toalumina, high k dielectrics with even substantially higher dielectricconstant would be desirable for replacement of the conventionallyemployed silica and/or silicon nitride layers.

Thus, e.g. titanium oxide (TiO) exhibits a dielectric constant ofgreater than 40. However, titanium oxide has only a low thermalstability. Further, in titanium oxide layers, high leakage currentsoccur, and the dielectric strength over the lifetime of the storagecapacitor is insufficient. These disadvantages basically also resultfrom most of the other contemplated binary metal oxides, such astantalum oxide having a dielectric constant of 25, or hafnium oxidehaving a dielectric constant of 25 to 30.

In order to achieve dielectric interlayers of a high k dielectric withimproved electrical and mechanical characteristics, therefore,increasingly, also mixed layers are employed.

Preferably, they are produced in form of stacked layers with the aid ofthe atomic layer deposition (ALD) method, in which the mixed layermaterials are applied separately from each other and mixed with asubsequent high-temperature process. Such an approach is described in US2005/0233598 A1, among other things. Here, as the layer sequence,preferably, a metal oxide layer and silicon oxide or silicon oxy nitrideare employed, respectively. However, the mixed layers proposed in US2005/233598 A1 can only difficultly be integrated into the standardsilicon planar technique methods. Further, the mixed layers also exhibithigh leakage currents and an insufficient dielectric strength. However,primarily, the mixed layers result in high tensile stresses on thesemiconductor surface and thus in strongly bending the semiconductorwafer.

There is a need for a method for producing a dielectric interlayer andfor a storage capacitor with such a dielectric interlayer, which exhibitimproved electrical and mechanical characteristics of the dielectricinterlayer.

SUMMARY OF THE INVENTION

According to one embodiment of the present invention, there is a methodfor producing a dielectric interlayer, especially for use in a storagecapacitor of a semiconductor memory cell, includes forming a first metaloxide layer, forming a metal nitride layer, oxidizing the metal nitridelayer to higher valency to form a second metal oxide layer; and heatingthe layer sequence to form a mixed layer from the first and the secondmetal oxide layer.

According to another embodiment of the present invention, there is amethod for producing a storage capacitor, especially for use in asemiconductor storage cell includes forming a first electrode layer,forming a dielectric interlayer on the first electrode layer and forminga second electrode layer on the dielectric interlayer. The dielectricinterlayer is formed including the steps of forming a first metal oxidelayer, forming a metal nitride layer, oxidizing the metal nitride layerto higher valency to form a second metal oxide layer and heating thelayer sequence to form a mixed layer from the first and the second metaloxide layer.

According to still another embodiment of the present invention, there isa storage capacitor, especially for use in a semiconductor storage cell,including a first electrode layer, a second electrode layer and adielectric interlayer disposed between the first electrode layer and thesecond electrode layer. The dielectric interlayer comprises a layersequence subjected to a temperature process, which has at least a firstmetal oxide layer and a second metal oxide layer formed by completelyoxidizing a metal nitride layer to higher valency.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is described in more detail below with referenceto the exemplary embodiments and drawings, in which:

FIG. 1 shows a circuit diagram of a DRAM cell.

FIG. 2 shows a schematic cross-section through a DRAM cell with astorage capacitor in form of a trench capacitor having a dielectricinterlayer according to the invention.

FIG. 3 shows an atomic layer deposition equipment for producing adielectric interlayer according to the invention.

FIGS. 4A to 4E show a method according to the invention for producing astorage capacitor within the bounds of a standard DRAM process flow.

It is to be noted, however, that the appended drawings illustrate onlytypical embodiments of this invention and are therefore not to beconsidered limiting of its scope, for the invention may admit to otherequally effective embodiments.

DETAILED DESCRIPTION OF THE INVENTION

The invention is explained with reference to trench capacitors, whichare produced within the bounds of the standard process sequence forforming DRAM storage cells based on silicon.

However, alternatively to trench capacitors, also other storagecapacitor forms can be realized in the manner according to theinvention, especially three-dimensional storage capacitor forms. Such athree-dimensional storage capacitor, which can be realized in the formaccording to the invention, is e.g. the stacked capacitor.

The formation of the trench capacitors according to the invention iseffected with the aid of the planar technique, which consists of asuccession of individual processes each acting on the entire wafersurface, wherein a local change of the substrate is purposefullyperformed by suitable masking steps. In producing highly integratedcircuits with storage capacitors, especially of DRAMs, a plurality ofstorage capacitors are formed at the same time. However, in thefollowing, the invention is only presented with respect to the formationof a single storage capacitor as a trench capacitor.

In DRAM memories, primarily single transistor cells are employed, thecircuit diagram of which is shown in FIG. 1. These transistor cellsconsist of a storage capacitor 1 and a selection transistor 2. Theselection transistor 2 is preferably formed as a field effect transistorand has a first electrode 21 and a second electrode 23, between which anactive region 22 is disposed, in which a current conducting channel canform between the first electrode 21 and the second electrode 23. Abovethe active region 22, an isolator layer 24 and a gate electrode areformed, which act like a plate capacitor, by which the charge density inthe active region 22 can be influenced.

The second electrode 23 of the selection transistor 2 is connected to afirst electrode 11 of the storage capacitor 1 through an electricalconnection 4. A second electrode 12 of the storage capacitor 1 is inturn connected to a conducting connection 5, which is preferably commonto all of the storage capacitors of the DRAM memory. Further, the firstelectrode 21 of the selection transistor 3 is connected to a bitline 6to be able to read in and out the information stored in the storagecapacitor in form of charges. Therein, the reading-in or reading-outoperation is controlled through a wordline 7, which is connected to thegate electrode 25 of the selection transistor 2, in order to establish acurrent conducting channel in the active region 22 between the firstelectrode 21 and the second electrode 23 of the selection transistor 2by applying a voltage.

In many cases, in dynamic storage cells, trench capacitors are employedas storage capacitors, since by the three-dimensional structure, asubstantial decrease of the storage cell area can be achieved. However,with increasing miniaturization of the storage cells, with a scalingsize below 100 nm, additional measures are required to be able tosatisfy three basic requirements to a dynamic storage cell in a DRAMmemory, namely a sufficiently great storage capacity of ca. 25 to 40 fF,which is required for reliable detection of the charge stored in thestorage capacitor, a packaging-dense and structurally friendly celllayout, which provides for minimum chip area and thus for reduced cost,as well as a high electrical performance, i.e. long retaining time ofthe charge in the storage capacitor, a high dielectric strength and highlifetime.

In order to provide for sufficient storage capacity with reduced storagecapacitor cross-section, the dielectric interlayer of silica and/orsilicon nitride conventionally employed in the storage capacitorsbetween the two capacitor electrodes is replaced by a high k dielectricwith a higher dielectric constant. This approach allows to downsize thecapacitor dimensions and to achieve a storage capacity required forreliable detection of the charge contained in the storage capacitor.

Preferred materials are binary metal oxides such as alumina. Alumina canbe integrated into the standard process sequence for forming storagecapacitors with the aid of the silicon planar technique in simplemanner. Moreover, alumina exhibits a high temperature stability, a greatdielectric strength as well as a low leakage current. With a dielectricconstant of 10, alumina provides for increased storage capacity per unitarea with respect to the conventionally employed silica and/or siliconnitride layers.

However, within the bounds of the desired downsizing of thesemiconductor storage cells, for increasing the storage capacity perunit area compared to alumina, high k dielectrics with an evensubstantially higher dielectric constant would be desirable. Possiblemetal oxide candidates are here e.g. zirconium oxide with a dielectricconstant of 11 to 25 and hafnium oxide with a dielectric constant of 25to 30. Also lanthanum oxide with a dielectric constant of 20 to 25 aswell as yttrium oxide with a dielectric constant between 11 and 12 canbe employed. However, with the above-mentioned metal oxides, there aredifficulties in integrating into the standard silicon planar process.Moreover, usually also high leakage currents occur, which result inshortened retaining time of the charge in the storage capacitor.

However, with all metal oxides, including alumina, which arecontemplated as high k dielectrics, it is problematic that such layersentail high tensile stresses on the semiconductor surface, which resultsin bending the semiconductor wafer. In employing three-dimensionalstorage capacitors within the bounds of the DRAM production, due to theincreased surface, deflections of several 100 μm can occur therein,which makes a further processing of the semiconductor substrate fordevice production within the bounds of the silicon planar technique,wherein the layers have to be consecutively applied precise in position,nearly impossible. Due to the high stress, there is also the danger thatthe semiconductor wafer breaks.

In order to improve the electrical and mechanical characteristics of thehigh k dielectrics, also mixed layers are increasingly employed. Herein,alumina compounds such as Hf—Al—O, Zr—Al—O or La—Al—O are considered.Further, high k dielectrics with a metal silicate compound such asHf—Si—O, Zr—Si—O or La—Si—O are contemplated. These alumina and metalsilicate compounds exhibit a dielectric constant of greater than 14 witha temperature stability up to 900° C. Such mixed compounds can beproduced with the aid of atomic layer deposition, wherein the individualmaterials are consecutively applied as thin layers and subsequentlymixed by the aid of a high-temperature process. However, also inemploying such mixed layers based on alumina and metal silicate, hightensile stresses on the semiconductor surface arise, which results ingreat deflection and makes further processing of the semiconductor wafernearly impossible. Furthermore, in employing these mixed layers asdielectric interlayers in storage capacitors, often reliability problemsalso occur. This relates especially to the dielectric strength and theleakage currents determining the retaining time of the charges in thestorage capacitor.

In order to reduce one or more of the above disadvantages in employinghigh k dielectrics as dielectric interlayers in storage capacitors, thedielectric interlayer according to the invention, especially for use ina storage capacitor in a semiconductor storage cell, has a layersequence of a first metal oxide layer and a metal nitride layer, whichis completely oxidized to higher valency to form a second metal oxidelayer. This layer sequence is then subjected to a high-temperatureprocess to form a mixed layer of the first metal oxide layer and thesecond metal oxide layer. The employment of a mixed layer of two metaloxide layers allows to optimally adjust the dielectric constant as wellas the remaining electrical characteristics, especially the leakagecurrent through the layer as well as the dielectric strength thereof incustomized manner by controlling the metal properties as well as thelayer thicknesses.

The application of the second metal oxide layer in form of a metalnitride layer, which is subsequently oxidized to higher valency,provides for widely preventing a stress of the semiconductor surface andthereby deflection. Namely, the oxidized metal nitride layer compensatesfor the tensile stress caused by the first metal oxide layer.

The first metal oxide is preferably formed from any of alumina, titaniumoxide, tantalum oxide, hafnium oxide, zirconium oxide and an oxide ofthe lanthanum group. Preferably, any of aluminum nitride, titaniumnitride, tantalum nitride, hafnium nitride, zirconium nitride siliconnitride and a nitride of the lanthanum group is employed as the metalnitride. Therein, preferably alumina is to be employed as the firstmetal oxide, and titanium nitride as the metal nitride, since aluminaand titanium nitride can easily be integrated into the standard siliconplanar process. Moreover, titanium nitride can be easily completelyoxidized to higher valency even at low temperatures. Thealumina/titanium oxide mixed layer formed by the high-temperatureprocess exhibits excellent mechanical and electrical characteristics,especially a low leakage current, a high dielectric strength as well asa high storage capacity per unit area. Further, by formation of themixed layer from an alumina layer and fully oxidized titanium nitridelayer, an excellent layer quality without stresses on the semiconductorsurface is achieved.

The layer stack for forming the mixed layer is preferably produced withthe aid of the atomic layer deposition (ALD) method. An ALD reactor ofthe showerhead type is shown in FIG. 3. The reactor is composed of areactor chamber 30 with a heatable support plate 31, on which asubstrate disk 32 to be processed is disposed. Further, a vacuum port 33is provided. Opposing the support plate 31 with the substrate disk 32, aso-called showerhead 34 with a plurality of parallel disposed gas exitopenings is disposed, which is connected to process gas conduits 35. Thevertical gas flow is characteristic of the showerhead type reactor.Alternatively, also a flow type reactor with a horizontal gas flow or abatch device can be employed.

In layer generation with the aid of the ALD reactor, the individualmaterials are consecutively introduced into the reactor chamber in formof precursors, wherein the chemical reaction for forming the layerexclusively occurs on the substrate surface. The introduction of thevarious reactive gases for forming the layer is separated from eachother by interposed inert gas steps, by which the reactor chamber ispurged before introduction of the next precursor. With the ALD method,extremely thin layers of only few atomic layers can be generated.

According to the invention, in formation of the high k dielectric mixedlayer, it is proceeded such that first the first metal oxide layer, thenthe metal nitride layer and subsequently once again the first metaloxide layer are applied. The individual layer thicknesses are preferablyin the range of 0.5 to 2 nm. The oxidation of the metal nitride layer tohigher valency is effected before application of the second layer of thefirst metal oxide layer with the aid of the oxygen precursor of thefirst metal oxide layer. For this, after application of the metalnitride layer, the oxygen precursor, e.g. ozone, is introduced into thereactor chamber and the substrate surface is heated to a temperature of300° C. to 400° C. At this temperature, then complete oxidation of thethin metal nitride layer to the second metal oxide layer is effected.Subsequently, then, further metal precursor for the first metal oxidelayer is introduced to form a second layer of the first metal oxidelayer.

After applying the entire layer sequence, then a high-temperatureprocess at preferably above 1000° C. is carried out to generate themixed layer. Instead of a separate high-temperature step, there is alsothe possibility to utilize one of the high temperature steps employedwithin the bounds of the subsequent device production for mixing. Theapproach according to the invention can be simply integrated into thestandard silicon planar process, wherein only a single reaction chamberis required by employment of the ALD process for layer production, whichmakes the process inexpensive and fast.

FIG. 2 shows a possible embodiment of a storage capacitor 1 according tothe invention as a trench capacitor. Therein, the trench capacitor 1 isformed in a preferably single-crystal silicon substrate 100. The siliconsubstrate 100 is preferably heavy p (p+) doped, e.g. with boron (B). Atrench 101 is realized in the silicon substrate 100, which is composedof a narrower upper trench region and a wider lower trench region. Sucha trench shape can be generated e.g. by a two-step etching processresulting in the bottle shape of the trench 101 shown in FIG. 2. Arounda first lower section of the trench 101, a highly n (n+) doped layer 103is formed, which for example is doped with arsenic. This n⁺ doped layerconstitutes the outer capacitor electrode of the trench capacitor 1 as aburied plate.

On the trench wall in the lower region, further, a dielectric layer 110composed of three layers is formed. Therein, the first layer is a 0.5 nmto 2 nm thick alumina layer, the second layer is a 0.5 nm to 2 nm thicktitanium oxide layer, which is formed by completely oxidizing a titaniumnitride layer to higher valency, and the third layer is a 0.5 nm to 2 nmthick alumina layer. The three layers are mixed by a high-temperaturestep. Instead of a three layer stack, there is the possibility to applystill further layers in alternating manner, wherein the first and thelast layer preferably always consist of alumina. Alternatively toalumina, also another one of the above-mentioned metal oxides can beemployed. The same applies to titanium nitride, which can also bereplaced by another one of the above-mentioned metal nitrides.

The trench 101 is then filled with an n⁺ doped filling layer 105,preferably a polysilicon layer, as the inner capacitor electrode. Theselection transistor 2 of the DRAM cell in the embodiment shown in FIG.2, has two diffusion regions 201, 202, which are generated by implantingdoping atoms into the silicon substrate 100 and separated by a channel203. The first diffusion region 201 serves as a first electrode layer 21of the selection transistor 2 and is connected to a bitline 6 through acontact layer 204. The second diffusion region 202 is connected to then⁺ doped filling layer 105, which constitutes the inner capacitorelectrode with the metal layer 104, through a capacitor terminal 205.The channel 203 is further separated by a dielectric layer 206 from agate electrode layer 207, which is part of a wordline 7.

In the upper region of the trench, an isolator layer 106 is providedadjoining the dielectric layer between the trench wall and the fillinglayer 105 of the trench capacitor 1. This isolator layer 206 preventsthat a parasitic transistor forms between the capacitor terminal 205 andthe buried plate 103 along the trench. Therein, preferably, silica isemployed as the isolator layer 106. In the silicon substrate, further ann⁺ doped well 107 is provided, which serves as a connection of theburied plate 103 to the buried plates of the further DRAM storage cells.For isolating the DRAM cells from each other, an isolating trench 108(STI isolation) is formed. The gate electrode layer 207 and the wordline7 are isolated from the bitline 6 in the contact layer 204 by an oxidelayer 208.

An operation of reading-in and reading-out the DRAM cell is controlledthrough the wordline 7 connected to the gate electrode layer 207 of theselection transistor to establish a current conducting channel 103between the diffusion regions 201, 202 by applying a voltage, such thatinformation in form of charge in the metal layer 104 in the trench 101can be read in and out through the terminal layer 205.

By the design of the dielectric interlayer according to the invention asa layer sequence subjected to a temperature process, which has at leasta first metal oxide layer and a second metal oxide layer, which isformed by complete oxidation of a metal nitride layer to higher valency,the electrical performance of the trench capacitor and thereby of theDRAM cell can be improved and at the same time the mechanical tensionand wafer bending can be decreased.

FIGS. 4A to 4E show a method for producing storage capacitors accordingto the invention as a trench capacitor with a dielectric interlayer,which is a mixed layer generated by a high-temperature process,consisting of a first Al₂O₃ layer, a fully oxidized TiN layer and asecond Al₂O₃ layer, within the bounds of a standard DRAM process.

As illustrated in FIG. 4A, in a first process step, the trenches (twoshown) for the trench capacitors are formed in a p⁺ doped siliconsubstrate 301. For this, an oxide layer 302 and a nitride layer 303 areconsecutively generated on the silicon surface. Subsequently, theregions of the trench capacitors are defined with a mask layer on thesilicon surface in known manner and trenches are etched by a first etchto a depth of up to 10 μm.

In a next process step, then, a thin etch-resistant layer 304 isdeposited to cover the upper region of the trench. The etch-resistantlayer 304 serves as an etch mask for a further etch step, in which thetrenches in the lower trench region are extended. A cross-sectionthrough the silicon wafer after this process step is illustrated in FIG.4B.

In another process sequence, the n⁺ doped buried plate 306 is formed.For this, the silicon substrate is doped with arsenic such that the n⁺doped layer 306 results around the extended region of the trench. Across-section through this silicon wafer after this process step isillustrated in FIG. 4C.

In a next process, now, a dielectric interlayer is formed in the manneraccording to the invention. This is preferably effected with an ALDreactor as is shown in FIG. 3. For this, in a first step, an aluminalayer 307 is grown in the lower section on the trench wall with a layerthickness of 0.5 to 2 nm. Subsequently, a titanium nitride layer isgenerated also with a thickness of 0.5 to 2 nm. The titanium nitridelayer is then completely oxidized to higher valency by introducing theoxygen precursor for generating the alumina layer and heating thesemiconductor wafer to a temperature of 300° C. to 400° C. Aftercompletely oxidizing the titanium nitride layer to the titanium oxidelayer 308, then again a further alumina layer 309 having a thickness of0.5 to 2 nm is generated. The cross-section through the silicon waferafter these process steps is illustrated in FIG. 4D. Subsequently, forgenerating a mixed layer, a high-temperature step, preferably at atemperature of 1000° C. and more, is then carried out in order toproduce an alumina/titanium oxide mixed layer.

In another process sequence, then, the inner capacitor electrode isproduced by subsequently filling the trench with a conductive fillinglayer 310, preferably n⁺ doped polysilicon. A cross-section through thesilicon wafer after this process step is illustrated in FIG. 3E. Thestorage capacitor can then be coupled to a selection transistor withinthe bounds of the known standard process sequence to produce DRAMstorage cells.

With the dielectric interlayer according to the invention in storagecapacitors comprising a layer sequence subjected to a temperatureprocess, which has at least a first metal oxide layer and a second metaloxide layer formed by completely oxidizing a metal nitride layer tohigher valency, the electrical characteristics of the storage capacitorconcerning dielectric strength, long-term stability and capacity, areimproved and at the same time, an improved integration into the standardprocess sequences for forming highly integrated circuits is achieved.

The formation of the dielectric interlayer from a layer stack consistingof two metal oxide layers, which are mixed by the aid of ahigh-temperature process, allows forming the dielectric interlayer incustomized manner with a high dielectric constant. At the same time,with such a mixed layer of two metal oxides, improved electricalcharacteristics, especially an increased dielectric strength over theindividual high k dielectric layer, can be achieved. Further, with sucha mixed layer, also the leakage current is reduced to a minimum.

By formation of the second metal oxide layer from a metal nitride layer,which is completely oxidized to higher valency, further, the stress ofthe semiconductor wafer can be substantially decreased due to thetensile stress caused by the first metal oxide layer, and thusdeflection of the semiconductor wafer can be prevented.

Therein, it is preferred to form the first metal oxide layer from any ofalumina, titanium oxide, tantalum oxide, hafnium oxide, zirconium oxideand an oxide of the lanthanum group, and to employ any of aluminumnitride, titanium nitride, tantalum nitride, hafnium nitride, zirconiumnitride, silicon nitride and a nitride of the lanthanum group as themetal nitride. A mixed layer produced from these materials exhibitsimproved electrical and mechanical characteristics.

This applies especially to a material combination, in which the firstmetal oxide layer is composed of alumina and the metal nitride layeroxidized to higher valency is composed of titanium nitride. Aluminaexhibits good integration into the standard process for producingstorage capacitors within the bounds of the silicon planar technique.The employment of oxidized titanium nitride as a further layer in thelayer stack provides for widely compensating for the high tensile stresscaused in production of the alumina layer on the substrate surface.Then, the subsequent mixing with the alumina layer provides for asubstantially higher dielectric constant with respect to a pure aluminalayer. The advantage of such a mixed layer of an alumina layer and atitanium oxide layer over a pure titanium oxide layer is a greatlyreduced leakage current as well as the distinctly higher dielectricstrength over the lifetime.

It is further preferred that the first metal oxide layer and the metalnitride layer are formed with the aid of an atomic layer deposition,wherein a further first metal oxide layer is applied onto the metalnitride layer. The oxidation of the metal nitride layer to highervalency before producing the further first metal oxide layer is effectedwith the aid of the oxygen precursor of the first metal oxide layer.This approach allows simple oxidation to higher valency without changeof equipment, which can be integrated into the standard silicon planarprocess in simple manner.

While the foregoing is directed to embodiments of the present invention,other and further embodiments of the invention may be devised withoutdeparting from the basic scope thereof, and the scope thereof isdetermined by the claims that follow.

1. A storage capacitor for use in a semiconductor storage cell,comprising: a first electrode layer, a second electrode layer, and adielectric interlayer disposed between the first electrode layer and thesecond electrode layer, wherein the dielectric interlayer comprises alayer sequence subjected to a temperature process, which has at least afirst metal oxide layer and a second metal oxide layer formed bycompletely oxidizing a metal nitride layer to higher valency.
 2. Thestorage capacitor according to claim 1, wherein the first metal oxide isany of alumina, titanium oxide, tantalum oxide, hafnium oxide, zirconiumoxide and an oxide of the lanthanum group, and the metal nitride is anyof aluminum nitride, titanium nitride, tantalum nitride, hafniumnitride, zirconium nitride, silicon nitride and a nitride of thelanthanum group.
 3. The storage capacitor according to claim 2, whereinthe first metal oxide is alumina and the metal nitride is titaniumnitride.
 4. The storage capacitor according to claim 1, wherein a trenchis realized in a semiconductor substrate, the first electrode layer inthe semiconductor substrate is formed around the trench, the dielectricinterlayer is realized on the trench wall, and the second electrodelayer is disposed on the dielectric interlayer.